In scroll compressor applications, particularly those in automotive environments, the rotational speed of the compressor and cooling load will vary over a wide operating range. Consequently, it is desirable to provide a configuration which maintains the discharge capacity constant irrespective of drive speed, and which evenly changes the discharge capacity as required by the cooling system. To meet these objectives, several approaches have been taken in the prior art.
A conventional variable capacity control type scroll compressor having a valve mechanism for opening and closing a plurality of bypass holes is disclosed in U.S. Pat. No. 5,451,146. That reference teaches a plurality of bypass holes that are disposed in side-by-side relationship through an end plate of a fixed scroll and communicate with the interior bypass passage of an elongated cylinder formed in the end plate. The bypass holes may be selectively opened to the bypass passage with a reciprocating plunger, to enable fluid bypass to a suction chamber formed in the housing. A control valve mechanism for opening and closing the bypass passage is located in the rear housing of the compressor. This structure has a disadvantage in that the arrangement of the bypass holes is asymmetric with respect to the fluid pockets undergoing compression, resulting in an unbalanced operating condition, reduced efficiency and greater noise.
Another variable capacity scroll compressor is disclosed in U.S. Pat. No. 5,074,760. This patent teaches a pair of bypass control valve mechanisms that control fluid bypass through bypass ports that are located symmetrically with respect to the fluid pockets being compressed. Another bypass hole is positioned proximal to the discharge port to enable the capacity control to range from 0 to 100%.
Yet another example of a variable capacity compressor is disclosed in Japanese Laid-open Patent 5-280476, wherein a cylinder having a coaxial, internally disposed valve plunger for sequentially closing a plurality of bypass holes communicating between the cylinder and a compression chamber is located in the end plate of the fixed scroll.
The structural configurations taught in the aforementioned prior art have several disadvantages. These include the relatively large number of parts and steps required during assembly resulting from utilizing numerous variable capacity components, thereby increasing the cost and weight of the overall assembly.
The variable capacity configurations taught in the '146 Patent and the Japanese Laid-open '476 Patent both have a disadvantage in that the bypass holes are opened asymmetrically (i.e., different positions) with respect to a pair of fluid pockets in the same compression stroke, thereby causing an uneven pressure balance, reduced efficiency and increased noise and vibration. In this regard, during high speed rotation, the bypass gas flow from the bypass hole communicating with the fluid pocket at the upstream side increases, and compressed gas is not fully returned to the suction chamber. Such gas flows partially back into the bypass hole communicating with the downstream side fluid pocket, causing increased pressure losses and reduced performance. Although the structure shown in the '760 Patent alleviates these problems, it utilizes multiple bypass valve assemblies to do it, consequently increasing manufacturing costs and lowering reliability.